Lecture 1: Precipitation and evaporation

Question 1

Describe the features of a catchment water balance, putting them in an equation equal to zero.

Answer 1

Precipitation - Evaporation - Runoff - storage in groundwater, soil and on surface.

Question 2

What are the three conditions for precipitation?

Answer 2

• Atmospheric Cooling
• Condensation on nuclei
• Growth of water/ice droplets

Question 3

Describe the process of atmospheric cooling that cause precipitation.

Answer 3

As the air is forced to rise, it expands, under reducing pressure and cooling, therefore decreasing its ability to hold water until at a certain point the air becomes saturated, and the vapour condenses and falls as rain.

Question 4

What are the three main types of precipitation that occur as the result of uplifted air masses?

Answer 4

• Frontal Precipitation
• Convective Precipitation
• Orographic Precipitation

Question 5

Describe Frontal Precipitation.

Answer 5

• Occurs by the convergence of two air masses, of different temperature and humidity along with a low pressure system.
• Where a cold front is advancing, and a warm front is retreating, the steep frontal surface of the cold air causes rapid uplift and intense rainfall of short duration.
• When a warm front is advancing, and a cold front retreating, the warm front has a less steep frontal surface, and light, prolonged rain/snow fall occurs ahead of the warm front.

Question 6

Describe convective precipitation.

Answer 6

Occurs when the earth’s surface heats the air above it, generating vertical air movement and leading to short-lived, locally intense rainfall.

Question 7

Describe orographic rainfall.

Answer 7

Occurs when a moist air mass encounters a barrier (i.e. mountain) on the earth’s surface, which forces it upward, resulting in precipitation on the windward (not leeward) side of the slope - ‘the rainshadow effect’.

Question 8

What is the global pattern of precipitation related to?

Answer 8

The patterns of evaporation and transport determined by the wind, as well as the vertical air movements in the atmosphere.

Question 9

Describe the global precipitation patterns in relation to high and low pressure systems.

Answer 9

Low precipitation under subtropical and polar highs.

High precipitation under ITCZ equatorial lows and subpolar lows.

Question 10

What type of data do rain gauges provide us with?

Answer 10

Direct, point measurements of rainfall at single point over a given time, with records available at varying intervals.

Question 11

What are the three methods of moving between point and areal rainfall data?

Answer 11

• Hypsometric
• Isohyetal
• Polygon

Question 12

Describe the Hypsometric method of moving between point and area data.

Answer 12

• Catchment is divided by hypsometric curves that correspond to elevations.
• Weighted average calculated from point rainfall between 2 elevations and the proportion of the catchment area between them.

Question 13

Describe the Isohyetal method of calaculating areal rainfall.

Answer 13

Catchment divided by isohyets - lines of equal rainfall between (a high number of) gauges - used to build a smooth rainfall surface by interpolating between points. Weighted based on the areas defined by isohyets and the average rainfall between them.

Question 14

Describe the thiession polygon method of moving between point and areal data.

Answer 14

Provides a weighted average of the rainfall given by a point measurement, based on the area which a single gauge represents - a function of the spacing between gauges.

Question 15

Describe weather radar estimates.

Answer 15

• Provide indirect measure of rainfall.
• Can estimate rainfall intensity using the reflectivity factor (the power reflected back to the radar from precipitation particles in the atmosphere.)
• Provides distributed / areal measurements of rainfall intensities.
• Identifies spatial and temporal variation in rainfall

Question 16

What is the relationship between condensation and evaporation in cooled air and warmed air?

Answer 16

Cooled air - Condensation > Evaporation

Warm air - Evaporation < Condensation

Question 17

Describe why the saturation deficit (E - e) is large in warm dry conditions and low in cold, moist conditions.

Answer 17

• E (the saturation vapour pressure) is large in warm conditions
• e (the actual vapour pressure) of water in the atmosphere is low in dry conditions

Question 18

What does daltons law tell us about evaporation from a free water surface?

Answer 18

It will be proportional to wind speed and vapour pressure deficit.

Question 19

What meteorological and physical factors govern evaporation from a free water surface?

Answer 19

• meteorological = Solar energy, humidity and diffusion processes.
• Physical = Salinity, water depth, size of surface.

Question 20

What meteorological and physical processes govern evaporation from soil?

Answer 20

• same meteorological ones as on free water surface (diffusion processes, solar energy and humidity)
• Physical factors = soil capillary characteristics, soil moisture content.

Question 21

What percentage of evaporation occurs over land, as opposed to sea? What percentage of rainfall occurs over land, as opposed to sea?

Answer 21

15% evaporation on land, 85% over sea.

23% rainfall on land, 77% rainfall over sea.

Question 22

Where is ocean evaporation highest?

Answer 22

The coastal waters of 40 degrees north and south.

Question 23

Describe the process of transpiration.

Answer 23

Water moves from soil to root to stem to leaf to being evaporated in the atmosphere.
Due to root pressure, capillary action and transpirational pull.

Question 24

What factors govern transpiration?

Answer 24

Meteorological factors - Energy, humidity, aerodynamic resistance
Vegetation characteristics - albedo, root control and stomatal control
Soil Factors

Question 25

Why might PET be high, but actual evaporation low?

Answer 25

PET estimated by amount of water

Vegetation types have different effects, forest = high evap, low albedo but grassland is opposite.

Question 26

Why is the global water balance affected by changes in surface cover?

Answer 26

Vegetation type affects evapotranspiration and infiltration rate.

Question 27

What equations estimate evapotransipration?

Answer 27

• Thornthwaite 1940/50s - empirical - based on annual and monthly heat index and average monthly temperature.
• Penman 1940s/60s - theoretical
• Penman-monteith 1965 - accounts for canopy (stomatal) resistance and included actual evaporation from a vegetation surface.